External flue heat exchangers

ABSTRACT

A heat exchanger is mounted external to a section of flue pipe or is an integral part of a section of flue pipe. The heat exchanger preheats a domestic hot water supply and boosts the return water temperature prior to reentry to the furnace coil. The heat exchanger reduces fuel use, pollution and wear of the furnace and burner. A typical heat exchanger installation includes an oil or gas burner located on a furnace or boiler having a flue pipe leading to a gaseous outlet, such as a masonry chimney. A short vertical flue section leads to a draft-regulating damper. The flue heat exchanger may be a coil of tubing wrapped around flue section, such that the tubing picks up heat from the heated flue gasses. Preheated water exits from the heat exchanger.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/404,073, filed Sep. 23, 1999.

FIELD OF THE INVENTION

The present invention relates to heat exchangers, which are mountedexternal to a section of flue pipe or are an integral part of a sectionof flue pipe.

BACKGROUND OF THE INVENTION

Heat exchangers are known, which direct water in a pipe through a flue.

U.S. Pat. No. 4,122,801 of Burns describes a heat exchanger which firstencircles water pipes in 2 circular rings around an exhaust flue, butthen directs the water into coils within the exhaust flue. The externalrings appear to be for positioning the water flow headers equally aroundthe flue, not to preheat the water.

U.S. Pat. No. 4,211,187 of Farris discloses an energy conservationsystem for heaters that uses a heat exchanger in a furnace chamber orduct.

U.S. Pat. No. 4,136,731 of DeBoer discloses a heat transfer apparatusfor supplementing a building heating and cooling system, using a heatexchanger in a furnace flue. DeBoer suggests an external heat exchanger,but provides no enabling details thereof.

Japanese patent no. JA0031286 of Satou discloses a heat transfer pipefor high temperature gases. It has a shell with multiple, non-axialconnected heat exchangers.

U.S. Pat. No. 6,068,047 of Buchhave describes a heat exchanger for asludge containment structure made of two rigid half shells, whereinsludge flows in the annular space enclosed by the shell halves and aninner sleeve in a spiral path external the spiral tubing with heatingfluid flowing therethrough.

U.S. Pat. No. 4,484,564 of Erickson disclose a water heater utilizingexhaust gases from furnaces or stoves, but the recovery is through acoil inside an exhaust flue.

U.S. Pat. No. 1,990,056 of Van Daam describes passing water through aspherical corrugated chamber.

U.S. Pat. No. 4,251,028 of Nicolai discloses a preheater with aninternal wall parallel to an external side wall with a water tight sealwith pressure restraining capability. It does not disclose a coil ofhollow heat conductive tubing enclosed within a substantially drycavity.

U.S. Pat. No. 3,896,992 of Borovina and U.S. Pat. No. 2,521,462 ofKinzelmann both disclose water heaters that pass water through a spiralcoil within an exhaust flue.

Canadian patent no. CA1271380 of Hampden describes a heat exchanger forair, not fluids, which is heated in a replaceable flue section. Hampdenuses a blower to draw air through fins in an annular space.

U.S. Pat. No. 4,037,567 of Torres proposes an exhaust flue over thewater heater having a spiral coil for heating water therein.

U.S. Pat. No. 4,120,267 of Wood describe a tube and plate heat exchangerwith water heating coils inside a chamber, such as a gas heat duct orflue.

German patent no. DE 3340281A of Grabietz describes a flue within ajacket wherein coiled water tubes wrap around the inner flue pipe.However, the space between the tubes positioned over the inner flue andwithin the outer jacket are filled with solid, cast molten metal,instead of fibrous insulation.

Soviet Union patent no. SU0779719 of Ukrorgtekhstroi discloses a heatexchanger with inner fins, not coils, which has heat resistant end capsenclosing a flue pipe. The fins are spiraled, but they are not hollowand do not carry fluid therein. Ukrorgtekhstroi does not contain tubingwrapped around a flue pipe.

Furthermore, U.S. Pat. No. 4,401,261 of Brown also discloses directingwater coils inside of flues.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a heaterexchanger for domestic hot water with ease of installation, maintenanceand removal.

It is another object of the invention to provide a heat exchanger thatis conveniently attached to an existing flue.

It is a further object of the present invention to preheat a domestichot water supply.

It is yet another object of the present invention to boost return watertemperature in a hydronic heating installation prior to reentry to afurnace coil.

It is still another object of the present invention to reduce fuel useand to reduce pollution and wear of the furnace and burner.

It is yet another object of the present invention to improve over thedisadvantages of the prior art.

SUMMARY OF THE INVENTION

In keeping with these objects and others, which may become apparent, thepresent invention includes heat exchangers mounted external to a sectionof flue pipe or as an integral part of a section of flue pipe. The heatexchanger preheats the domestic hot water supply and boosts the returnwater temperature in a hydronic heating installation prior to reentry tothe furnace coil. The heat exchanger reduces fuel use and reducespollution and wear of the furnace and burner.

In a preferred embodiment, a split heat exchanger formed from two halfshells forms a heat exchanger with conduits having flow reversals toenhance heat transference.

A typical heat exchanger installation includes an oil or gas burnerlocated on a furnace or boiler having a flue pipe leading to a gaseousoutlet, such as a masonry chimney. A short vertical flue section leadsto a draft-regulating damper. The flue heat exchanger may be a coil ofcopper tubing wrapped around the flue section, which picks up heat fromthe heated flue gasses. The cold water source is coupled to a shortlength of convoluted flexible tubing with coupling flanges therebyallowing water to travel to and from the flue mounted heat exchanger.

In another embodiment two flue heat exchangers communicate with coldwater entering the horizontal heat exchanger which is wrapped around aflue section having a mixture of hot flue gasses and some make-upambient air from the draft regulating damper. This heat exchanger isplumbed in series with another heat exchanger wrapped around thevertical section of flue pipe below a damper. The second heat exchangeragain increases the water temperature prior to entering the furnace hotwater coil.

In another embodiment a heat exchanger is prefabricated as a standardflue section and substitutes for a length of flue. This heat exchangerhas a central flue pipe section with heat exchanger tubing wrappedaround its periphery. A tubular shell encases the tubing with openingsallowing for both the water inlet and water outlet coupling flanges. Ahighly conductive conformable material fills the empty spaces within theshell to increase heat transfer.

In another embodiment, a hydronic heating system is a hydronic loopcirculated by circulator pump forcing water into heating coil inside aboiler or furnace. In this system, it first flows through heat exchangerwhere it picks up waste heat from the flue.

In a further preferred embodiment a preformed heat exchanger coil iswrapped around a cylindrically shaped sheet larger in diameter than asection of flue. This sheet is not totally enclosed, but it has a smallgap along its length.

In yet another embodiment, two coiled heat exchanger conduits areinterleaved together around a flue pipe.

Each embodiment of the heat exchanger may include a safety pressurerelief valve through which preheated water may exit.

Often when two or more different types of metals contact each other, themetals deteriorate, corrode or weaken at the point of contact.Therefore, each embodiment of flue heat exchanger may also beconstructed such that both the metal tubing and the section of fluepiping that the metal tubing is wrapped around are made from the sametype of metal, preferably copper. In addition, each embodiment thatcontains this variation may also include gaskets located at each end ofthis flue section (preferably copper), such that the gaskets preventdirect contact between this flue pipe section and the flue pipe sectionmade from a different type of metal.

In a further embodiment of this invention, the heat exchanger comprisesof two identical sections, each essentially of the shape of a halfcylindrical tube, with an internal diameter which matches the externaldiameter of a flue pipe. By just assembling the two halves around anexisting flue pipe in-situ and bolting them together through flanges, itis unnecessary to disconnect flue pipe sections with the attendantdebris usually encountered. Thus this embodiment is easier to install,lower cost (since the original flue pipe is retained), easier topackage, and adaptable to single or split use, such as domestic hotwater and hydronic heating.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can best be understood in conjunction with theaccompanying drawings, in which:

FIG. 1 is a side elevation view of a flue heat exchanger installation ofthe present invention;

FIG. 2 is a side elevation view of an installation using two flue heatexchangers;

FIG. 3 is a perspective view of an alternate embodiment for a flue heatexchanger;

FIG. 3a is a perspective view of an alternate embodiment of a heatexchanger cover;

FIG. 4 is a side cross sectional view of the embodiment shown in FIG. 3;

FIG. 5 is a plumbing diagram of a flue heat exchanger used for hydronicheating;

FIG. 6 is a side elevation view of a preferred embodiment of a flue heatexchanger;

FIG. 7 is an end view of the embodiment shown in FIG. 6;

FIG. 8 is a side elevation view of an interleaved heat exchangerinstallation;

FIG. 9 is a perspective view of another embodiment of a flue heatexchanger;

FIGS. 9A and 9B are respective end and cross sectional views of theembodiment shown in FIG. 9;

FIG. 10 is a side end view of an alternate embodiment for a splitexternal heat exchanger assembled over a flue pipe;

FIG. 10A is a side internal elevation view thereof showing a transverseserpentine heat exchanger element;

FIG. 10B is a perspective internal view thereof showing a longitudinalserpentine heat exchanger element;

FIG. 10C is a side internal view in cross section showing the fluid paththrough a formed tank heat exchanger element;

FIG. 10D is an end view in cross section of nested sections of the splitheat exchanger in a shipping carton;

FIG. 10E is an end view in cross section of reversed nested sections ofthe split heat exchanger in a shipping carton;

FIG. 10F is a side elevation view of a split heat exchanger thereofinstalled and plumbed for parallel flow of both sections;

FIG. 10G is a schematic representation of plumbing for a seriesconnection of both sections of the split heat exchanger thereof; and,FIG. 10H is a schematic representation of plumbing for a separate use ofeach section thereof.

DETAILED DESCRIPTION OF THE INVENTION

For ease of installation, maintenance, and removal, the heat exchangersof this invention are mounted external to a section of flue pipe or arean integral part of a section of flue pipe.

Although the primary application is the preheating of a domestic hotwater supply, a secondary application is the boosting of return watertemperature in a hydronic heating installation prior to reentry to thefurnace coil. In either case, the objectives are to reduce fuel use andto reduce pollution and wear of the furnace and burner.

FIG. 1 shows a typical installation showing an oil or gas burner 6 on afurnace or boiler 5 with flue pipe section 2 leading to masonry chimney3. A short vertical flue section 15 leads to a draft regulating damper4. The flue heat exchanger 1 includes a coil of heat conductive tubing,such as copper tubing, which is wrapped around flue section 2. Flue heatexchanger picks up heat from the heated flue gasses within flue section2. The cold water source 7 is coupled to a short length of convolutedflexible tubing 9 with coupling flanges 8 at either end, which couplethe cold water through heat exchanger 1. The exit of preheated waterfrom heat exchanger 1 is coupled to another short length of flexibletubing 9 and then coupled to a short length of pipe via coupling flanges8.

This leads to a safety pressure relief valve 10 and isolation valve 11(normally open) which couples the preheated water to a domestic supplypipe 12 (optional) and to the boiler 5 hot water coil intake 13. Pipe 14is the normal domestic hot water supply line from the hot water coil.

The need for safety pressure relief valve 10 is predicated on rareevents, which could conspire to cause boiling or excessive pressure inheat exchanger 1. While flue surface temperatures above 212 degrees F.are sometimes encountered, water at normal supply pressure (above 35psig) requires a flue temperature over 280 degrees F. to present adanger of boiling. In the event of a defective well pump or control, thewater pressure could be abnormally low. Likewise, an improperly adjustedfuel burner could produce abnormally high flue temperatures approaching300 degrees F. The combination of such events may result in excessivepressures, hence the pressure relief valve. This danger is morepronounced in situations with a common boiler supplying heat and hotwater since it would be more likely for the burner to be on while thereis no call for domestic hot water; and there would be no cooling waterflow through the heat exchanger.

FIG. 2 shows an installation using two flue heat exchangers 1 and 20 ina single installation. The coldest water enters the horizontal heatexchanger 1, which is wrapped around flue section 2 having a mixture ofhot flue gasses and some make-up ambient air from draft regulatingdamper 4. Heat exchanger 1 is plumbed in series with heat exchanger 20,which is wrapped around the vertical section 2 a of flue pipe belowdamper 4. This section of flue pipe 2 a has a surface temperature higherthan flue section 2 since it is closer to furnace 5 and only has hotflue gasses within. Therefore, heat exchanger 20 further boosts watertemperature prior to entering the furnace 5 hot water coil.

FIGS. 3 and 4 show two views of an alternate embodiment 25 of the heatexchanger of this invention. This heat exchanger 25 is prefabricated asa standard flue section length L. It would be simply substituted for alength of flue. Heat exchanger 25 includes a central flue pipe section26 with heat exchanger tubing 30 wrapped around its periphery. A tubularshell 31 with heat resistant end caps 32 encases the tubing 30 withopenings for inlet 28 and outlet 27 extensions terminating in couplingflanges 29. A highly conductive conformable material 33 such as copperor aluminum wool is forced between flue 26 surface and coils 30 andgenerally fills the empty spaces within shell 31 to increase heattransfer. Further efficiency is achieved if shell 31 is a thermalinsulator such as a fiberglass liner within a plastic hard shell.

FIG. 3a shows a removable embodiment of tubular shell 31 with openingsfor inlet 28 and outlet 27 extensions. A highly heat conductiveconformable material 33 such as copper or aluminum wool is secured tothe entire inner wall of tubular shell 31. Clasps 80 are placed alongthe outside of the surface of tubular shell 31 thereby allowing a meansto secure the tubular shell around both flue pipe 26 and heat exchangercoil 30.

FIG. 5 shows the plumbing hook-up for the use of an external flue heatexchanger 1 in a hydronic heating system using fin tube heating elements44. The basic circuit is a hydronic loop circulated by circulator pump40 forcing water into heating coil 41 inside boiler (furnace) 5 thenleading to expansion tank 42 and further to a parallel arrangement ofzone valves 43 through fin tube sections (baseboard hot water roomunits) 44 and through return manifold 45. In systems without heatexchanger 1, this return flow would be directly plumbed to the intake ofcirculator pump 40. In this system, the return flow first flows throughheat exchanger 1 where it picks up waste heat from the flue.

The preferred embodiment shown in FIGS. 6 and 7 shows a preformed heatexchanger coil 1 wrapped around a cylindrically shaped sheet (or sleeve)of metal 55 such as copper which is slightly larger in diameter than asection of flue. Sheet 55 is not totally enclosed, but it has a smallgap along its length. The cylindrically shaped sheet is soldered orbrazed to the copper tubing 1 for approximately three-quarters of itscircumference to enhance heat transfer. The region indicated by 90degrees in FIG. 7 is not bonded to tubing 1, thereby allowing the sleeveto open and enclose the flue. Three sets of clamping tabs 58 are locatedat each end and in the middle in a gap between adjacent coils of tubing1. Bolts 56 and nuts 57 are used through tabs 58 to insure sheet 55 fitssecurely around a flue section 2.

An alternate embodiment specifically for dual-use boiler installationsis shown in FIG. 8. An interleaved coil flue heat exchanger 70 is shownwrapped over flue pipe section 2. It includes two separate conduits.Coil 71 plumbed into the domestic hot water return 13 and coil 72plumbed into the hydronic heating return line 73. The method for formingthis flue heat exchanger 70 is to coil two lengths of bendable tubingtogether resulting in the interleaved coils of sections 71 and 72. Theplumbing is straightforward with the outlet end of coil 72 leading tocirculator pump 40 through conduit 74 thus boosting the temperature ofheating water that has been cooled by its flow through the various roomhydronic heating units such as baseboard fin tube units or radiators.Similarly, the cold supply water at 7 uses the interleaved flue heatexchanger 70, a single section of flue pipe 2 can be used to recoverwaste heat year round regardless of whether the burner 6 is being firedto generate heat, hot water, or both. It is highly likely thatcirculator pump 40 will be running or that domestic hot water demandwill occur while burner 6 is active or while flue pipe 2 is still hotfrom a recent firing. For dual-use installations, this maximizes theflue waste heat recover on a seasonal basis.

FIGS. 9, 9A and 9B show an alternate embodiment of a flue heat exchangerwith particular gaskets 85, 86. This is because when two or moredifferent types of metals contact each other, the metals may sometimesdeteriorate, corrode or weaken at the point of contact. Therefore, whileit is preferable that a flue heat exchanger may be constructed such thatboth the metal tubing 88 and the section 87 of flue piping that themetal tubing 88 is wrapped around are made from the same type of metal,preferably copper, modifications must be made if different metals are incontact with each other.

Therefore, FIGS. 9, 9A and 9B show a flue piping section 87 of a flueheat exchanger having metal tubing 88 wrapped around it, with fluepiping section 87 having gaskets 85, 86 located at each end of fluepiping section 87(preferably copper), such that the gaskets 85, 86prevent direct contact between this flue pipe section 87 and thepermanent flue pipe section which may be made from a different type ofmetal other than copper, to which flue pipe section 87 is attached.

The split external heat exchanger is yet another embodiment that isespecially adapted for the “do-it-yourself” market as exemplified by thelarge chains of home improvement outlets. This design is simply placedover an existing flue pipe and bolted together. The flue pipe need notbe disassembled. It also fits over seams in flue pipes so that the heatexchanger section can be longer than an individual section of flue aslong as a total straight section of flue of sufficient length isavailable.

FIG. 10 shows a split heat exchanger 100 assembled over existing fluepipe 107. It includes two identical halves with heat exchanger element102 formed within shell 101. Semi-circular end pieces and a layer ofthermal insulation between heat exchanger element 102 and shell 101 canbe added to reduce heat loss to ambient air, but they are not essentialto good operation. Flanges 105 with multiple holes and bolts 106 areused to hold the two halves together tightly around flue 107. Nipples103 and 104 are inlet and outlet (or reversed) nipples to permit liquidflow through heat exchanger elements 102. Three separate embodiments ofheat exchanger elements 102 are described.

FIG. 10A shows element 111 which includes a separate hollow fluid flowchamber, such as a transverse serpentine tubing section conforming tothe interior of shell 101 with attachment holes 110 in flanges 105. Heatexchanger element 111 with its many flow reversals creates much flowturbulence which enhances heat transfer efficiency, however eachtransverse section must be shaped in a circular manner to conform toshell 101.

FIG. 10B shows a second type of heat exchanger element 115 wherein theseparate hollow fluid flow chamber is a longitudinal serpentine tubingsection which is easier to conform to shell 101 since the straight pipesections do not require forming.

A third heat exchanger element 120 shown in FIG. 10C includes a shallowtank with curved inner and outer surfaces and circular end pieces. Theinner surface conforms closely to the outer diameter of a flue pipeenhancing heat transfer while the outer surface conforms to shell 101.Each separate hollow, fluid flow chamber comprises a fluid flow pathformed by internal straight baffle plates 121 directing liquid flow in areversing path, as shown by the arrows to increase turbulence andprevent stagnant regions.

While FIG. 10C shows the hollow fluid flow path to be longitudinallyoriented similar to the longitudinal serpentine fluid flow path shown inFIG. 10B, alternatively curved baffle plates (not shown) can be used tosimulate a transverse fluid flow path, similar to the transverse fluidflow path of FIG. 10A.

Since the intent is to market split heat exchangers 100 through retailhome improvement centers, any features that enhance packaging andshipping is an asset. FIG. 10D shows a top view of the two halves ofsplit heat exchanger 100 partially nested in a compact configuration inshipping carton 125.

An alternate configuration using reverse nesting in shallower box 126 isshown in FIG. 10E.

Because split heat exchanger 100 has two independent heat exchangerelements 102, these can be plumbed in a number of ways to achievedesired results. It can also be appreciated that multiple split heatexchangers 100 can be used on a single long flue and plumbed as a singlesystem or as individual heat exchangers.

FIG. 10F shows how split heat exchanger 100 can be connected so thateach section is in parallel by connecting both nipples 104 toY-connector 130 and both nipples 103 to a second Y-connector 130.

FIG. 10G shows a series connection which would produce a higher outputtemperature by connecting the input to nipple 104 of the first side andthen connecting nipple 103 of the first side to nipple 103 of the secondside. Output flow is then from nipple 104 of the second section.

FIG. 10H shows a plumbing configuration where both sections are usedindependently. In fact, the left section with input A at nipple 104 andoutput B at nipple 103 can be used for domestic hot water, while theright section with input C at nipple 103 and output D at nipple 104 canbe used to boost boiler return water in hydronic heating.

It is further noted that other modifications may be made to the presentinvention, without departing from the scope of the invention, as notedin the appended claims.

I claim:
 1. A heat exchanger for preheating domestic water bytransferring heat from a flue containing combustion gases flowingtherethrough comprising: said heat exchanger replacing a portion of saidflue and having a single, tube for connecting the replaced portion ofsaid flue to allow flow of combustion gases therethrough; said singletube being axially oriented with said flue; said single tube beingsmooth and unencumbered in an interior portion thereof; said single tubebeing enclosed by a cylindrical shell spaced from said single tube withcaps at upstream and downstream ends of said shell for fully enclosing aspace within said shell surrounding said single tube; helical tubingbeing located within said shell, said helical tubing having an inletadjacent a downstream end of said helical tubing and an outlet adjacentan upstream end of said helical tubing; said shell having a pair ofrespective openings accommodating said inlet and said outlet of saidhelical tubings therethrough; said helical tubing within said shellcoiled around said tube, said helical tubing connecting said inlet andoutlet within said shell; a highly conductive conformable looseaggregate of fibrous material filling spaces within said shell betweenand around said helical tubing to increase heat transfer from combustiongases within said single tube to said helical tubing; said shell beingmade from insulating material and said end caps being heat resistant;and means for circulating water through said helical tubing from saidinlet to said outlet of said helical tubing within said shell forheating said water.
 2. The heat exchanger as recited in claim 1 in whichsaid shell is removable and has facing edges extending axially alongsaid shell passing through each of said inlet and outlet of said helicaltubing with clasps for joining said edges together.